Fuel cells are expected to be widely used in the future since their power generation efficiency is high, and their load to the environment is light. Particularly polymer electrolyte fuel cells are expected to be widely used for movable bodies such as automobiles, or as distributed power generation system, or cogeneration systems for home use, since their power density is high and their operating temperature is low, whereby downsizing can be carried out.
A cross-sectional view of a single cell for conventional fuel cells is shown in FIG. 17. In FIG. 17, a single cell 1 for fuel cells has a polymer electrolyte membrane 11. This polymer electrolyte membrane 11 usually has a thickness of from about 20 to 120 μm, and a cation exchange membrane made of a perfluorocarbon polymer having chemically stable sulfonic groups is used for it.
Further, two catalyst layers 27 and 28 each containing a metal catalyst are bonded to both outer surfaces 11a of the polymer electrolyte membrane 11. These catalyst layers 27 and 28 are formed on the center portion of the polymer electrolyte membrane 11, and a portion not bonded to the catalyst layers 27 and 28 is left along its periphery.
Further, a membrane-catalyst layer assembly 31 is constituted by such a polymer electrolyte membrane 11 and catalyst layers 27 and 28, and gas diffusion layers 33 and 34 are respectively disposed on both outer surfaces 31a of the membrane-catalyst layer assembly 31 on the side of the catalyst layers 27 and 28. In order to conduct electrons entering into or leaving from the catalyst layers 27 and 28, these gas diffusion layers 33 and 34 have sizes which are the same as or larger than the sizes of the catalyst layers 27 and 28, and they are formed from e.g. carbon paper or carbon cloth.
However, as shown in FIG. 18, the gas diffusion layers 33 and 34 may be disposed to cover not only the surfaces of such catalyst layers 27 and 28 but also their sides. In such a case, the gas diffusion layers 33 and 34 may also be contacted with the polymer electrolyte membrane 11 at contact surfaces 33a. 
Further, a membrane-electrode assembly 37 is constituted by the membrane-catalyst layer assembly 31 and the gas diffusion layers 33 and 34, and, on both outer surfaces 37a of the membrane-electrode assembly 37, on the side of the gas diffusion layers 33 and 34, gas channels 47 and 48 are formed between them and separators 41 and 42.
Here, the separators 41 and 42 have such sizes as to cover the entire surface of the polymer electrolyte membrane 11, and concave grooves 45 and 46 are engraved at the respective portions facing the catalyst layers 27 and 28, so that when the separators 41 and 42 and the membrane-electrode assembly 37 are fastened, such grooves 45 and 46 will form the gas channels 47 and 48.
Furthermore, at the portions of the separators 41 and 42 facing the portions of the membrane-electrode assembly 37 where the catalyst layers 27 and 28 are not bonded, gaskets 53 and 54 are located for sealing so that a fuel gas and an oxidant gas will not leak to outside, and when the separators 41 and 42 and the membrane-electrode assembly 37 are assembled, such gaskets are interposed between the separators 41 and 42 and the polymer electrolyte membrane 11, so that the gas channels 47 and 48 are sealed against outside.
As described above, a single cell 1 is constructed as a minimum unit for power generation by a fuel cell, and in a case of using such single cells 1 for fuel cells, a plurality of single cells 1 may be used as laminated or stacked so as to generate a practical voltage.
In such a construction, hydrogen is supplied to the anode (catalyst layer 28) side of the single cell 1. On the other hand, oxygen or air is supplied to the cathode (catalyst layer 27) side. At that time, hydrogen, oxygen and air are supplied through the gas channels 47 and 48. As a result, a reaction of H2→2H++2e− takes place on the anode side. H+ (proton) produced on the anode side transfers to the cathode side through the polymer electrolyte membrane 11, and e− (electron) transfers to the cathode side via an external circuit. On the other hand, on the cathode side, the proton transferred from the anode side through the membrane, the electron transferred via the external circuit and oxygen supplied are reacted, whereby a reaction represented by ½O2+2H++2e−→H2O takes place.
Thus, in a fuel cell having the single cell 1, chemical energy can be converted to electric energy. In order for the proton to pass through the polymer electrolyte membrane 11, the polymer electrolyte membrane 11 is required to be in such a state that it holds water. Therefore, in order to carry out such a reaction efficiently, gases to be supplied to the anode and the cathode are humidified and then supplied thereto.
However, in a fuel cell constructed as described above, at end portions 31b of electrode-catalyst layers shown by a dotted line circle in FIG. 17, at contact surfaces 33a from end portions of catalyst layers 27 and 28 to end portions of the gas diffusion layers 33 and 34 in FIG. 18, or at end portions 30 (portions in contact with the polymer electrolyte membrane 11) of the gaskets 53 and 54 at the side of the catalyst layers 27 and 28, there has been a problem such that the polymer electrolyte membrane 11 tends to have holes, or short circuiting of electrodes is likely to occur though the reason is not clearly understood.
As causes of troubles at the end portions 31b of the electrode catalyst layer in FIG. 17, it is presumed that the pressure at the time of bonding electrodes is exerted on the end portions so strongly that the polymer electrolyte membrane 11 is likely to be damaged, or a creep phenomenon occurs at the end portions 31b of the electrode catalyst layer, since the pushing pressure is exerted from both sides of the membrane-electrode assembly 37 also during the operation, whereby gas leakage increases and a local burning reaction takes place, to cause membrane decomposition or short circuiting. Accordingly, it is considered advisable that the portions where the polymer electrolyte membrane 11 and the catalyst layers 27 and 28 are bonded at the end portions 31b of the electrode catalyst layer, have a reinforced structure.
As a means for solving the above problems, a membrane-electrode assembly has been proposed which has a structure such that reinforcing frames made of polymer sheets are interposed between the end portions of electrode catalyst layers and the polymer electrolyte membrane (Patent Documents 1 and 2). However, in the case of this membrane-electrode assembly, there is a problem that membrane damage occurs in the vicinity of end portions inside of the reinforcing frames, though membrane damage at the end portions of the electrode catalyst layers can be suppressed.
Accordingly, a membrane not substantially containing a reinforcing material in the vicinity of the center of the conductive portion of the polymer electrolyte membrane and containing a reinforcing material such as fiber, fabric, fibril or porous membrane in the vicinity of the boundary between the conductive portion and the non-conductive portion around it, has been proposed (see Patent Documents 3 and 4). However, in the case of this membrane, in the vicinity of the center, its strength is insufficient though resistance is low, and the gas permeability of the reinforced portions is suppressed but the suppression is still insufficient, and during a long-term operation, a defect of the membrane or short circuiting was likely to occur in the vicinity of the end portions of the electrode catalyst layers.
Further, a membrane-electrode assembly has been proposed, which is prepared in such a manner that holes of 3 mmΦ are formed in 7 rows×7 columns on a polytetrafluoroethylene (PTFE) film so that the distance between the centers of adjacent holes is 6 mm, a perfluorosulfonic acid polymer is impregnated in the holes and dried to prepare a membrane having a conductive portion with an area of 39 mm×39 mm, and then electrodes of 50 mm×50 mm are bonded on both sides of the membrane (see Patent Document 5). However, in such a case, there was such a problem that the proton conductivity in the vicinity of its center is insufficient though proton conductivity at the peripheral portions is low, whereby the power generation property is low.
On the other hand, as causes of troubles on the contact surfaces 33a from the end portions of the catalyst layers 27 and 28 to the end portions of the gas diffusion layers 33 and 34 in FIG. 18, it is presumed that the pressure at the time of bonding the gas diffusion layers 33 and 34 is exerted on their end portions so strongly that the polymer electrolyte membrane 11 is likely to be damaged, the pushing pressure is exerted on the membrane-electrode assembly 37 also during the operation, whereby the gas diffusion layers 33 and 34 having relatively large irregularities on their surfaces are pushed at portions in direct contact with the polymer electrolyte membrane 11 to decrease the membrane thickness, and outside of the outer edges of the catalyst layers 27 and 28, supplied gas is not consumed and is likely to remain, whereby the gas concentration becomes high and the gas permeability becomes high.
As a result, it is considered that a burning reaction locally takes place, and membrane decomposition or short circuiting occurs. Therefore, a polymer electrolyte membrane having such a structure that portions of the contact surfaces 33a are reinforced, is considered to be preferred.
As a means for solving the above problems, a membrane-electrode assembly has been proposed, which is prepared in such a manner that a sealing material of tetrafluoroethylene/propylene copolymer is applied on peripheral portions of the gas diffusion layers and dried to prepare assistant gaskets having a width of from 2 to 10 mm and a thickness of 60 μm, and catalyst layers are formed inside of such gaskets and then bonded with an ion exchange membrane (see Patent Document 6).
However, in the case of this membrane-electrode assembly, it is difficult to prepare the assistant gaskets and to form catalyst layers precisely in it. Therefore, there is such a problem that the catalyst layers are likely to overlap on the assistant gaskets to form defects.
Further, a membrane-electrode assembly has been proposed, which is prepared in such a manner that on center portions of gas diffusion layers, catalyst layers with a smaller area are respectively applied and dried, followed by bonding with an ion exchange membrane having fluororesin sheets with an opening of the same size as the catalyst layers bonded thereto (see Patent Document 7).
However, misalignment may occur at the time of bonding the above gas diffusion layers with the ion exchange membrane, and the catalyst layers and the fluororesin sheets may overlap to form defects.
Further, a membrane-electrode assembly has been proposed, which is prepared in such a manner that a fluororesin sheet having an opening with a certain size is bonded on each side of an ion exchange membrane, a catalyst layer with the same size as the opening is applied to the opening and dried, and then a gas diffusion layer larger than the opening is bonded to the catalyst layer (see Patent Document 8).
However, it is difficult to apply the catalyst layer with the same size as the opening, and the catalyst layer and the fluororesin sheet are likely to overlap to form defects.
Further, as a cause of the trouble of the gaskets 53 and 54 at the end portions 30 on the side of catalyst layers 27 and 28, it is presumed that the pressure at the time of bonding of the gaskets 53 and 54 is exerted on the end portions so strongly that the polymer electrolyte membrane 11 is likely to be damaged, the pushing pressure is exerted on the gaskets 53 and 54 also during the operation, whereby the gaskets 53 and 54 having relatively large irregularities on their surface are pushed at portions in direct contact with the polymer electrolyte membrane 11 to decrease the membrane thickness, and outside of the outer edges of the catalyst layers 27 and 28, supplied gas is not consumed, and is likely to remain, whereby the gas concentration becomes high, and the gas permeability becomes high.
As a result, it is considered that a burning reaction locally take place, and a membrane decomposition or short circuiting occurs. Therefore, a polymer electrolyte membrane is considered to be preferred, which has such a structure that the vicinity of the inner portions 30 of the gaskets 53 and 54 is reinforced.
As a means for solving the above problems, a membrane-electrode assembly has been proposed, which has such a structure that reinforcing frames of a polymer sheet are interposed between gaskets and a polymer electrolyte membrane (see Patent Document 2).
However, in the case of such a membrane-electrode assembly, there is a problem that the membrane is damaged in the vicinity of the end portions inside of the reinforcing frames, though the membrane damage at the end portions of gaskets can be suppressed.
Also with respect to such problems, it is conceivable to solve them by membranes as described in the Patent Documents 3 to 5. However, also in such cases, there will be the same problems as the problems caused by the membranes as described in the above-mentioned Patent Documents 3 to 5.
Patent Document 1: Japanese Patent Publication 3245161 (claim 1)
Patent Document 2: Japanese Patent Publication 3368907 (claim 1)
Patent Document 3: JP-A-2000-260443 (claims 1 and 3)
Patent Document 4: JP-A-8-259710 (Example 3)
Patent Document 5: JP-A-2000-215903 (Example 3)
Patent Document 6: JP-A-7-220742 (Example 1)
Patent Document 7: JP-A-10-154521 (Example 1)
Patent Document 8: JP-A-10-308228 (Example 3)